1. Field of the Invention
The present invention relates to a multi-stage dynamic braking resistor network and associated control circuitry for absorbing and dissipating energy that is put onto a system bus and a method thereof.
2. Description of the Background Art
Conventional braking impedances, which are typically referred to as braking resistors, have been applied to power systems, for example, remote generation locations. The braking resistor provides a shunt resistance path to ground when coupled with the power system via a switch.
FIG. 1 is a perspective view of an example of a conventional dynamic brake resistor 2. The dynamic brake resistor 2 can be an assembly of grid elements, for example. Each assembly comprises a first stack 3 and a second stack 5, with each stack having an upper layer 7, 9 and a lower layer 11, 13. Each layer is formed, for example, from a plurality of stamped metal sheet units having a serpentine configuration, which are welded at their ends to adjacent units to form a series resistance element.
Typically, braking resistors increase a load on a system bus, whereby an increased load on the system bus will absorb the excessive energy generated by a rotary actuator or an electric motor. In other words, during a regeneration mode of operation, a motor acts as a generator to supply power to the system. For instance, when driving power is disconnected from a motor or actuator, the motor or actuator continues to rotate and thereby regenerated energy is placed onto the system bus. In some systems, this regenerated energy is returned to the power source. When the power source is not receptive to such regenerative energy, an overvoltage will occur and possible damage could happen to the system. To prevent such overvoltage, a dynamic brake resistor can be used to absorb/dissipate the regenerated power by converting the regenerated power to thermal energy, e.g., heat.
Some braking resistors have a mechanical switch that couples the resistor to the power system for a predetermined time following a detection of a system disturbance. These mechanical switching braking resistors, however, are abruptly removed from the system bus by opening the mechanical switch. After removal from the system bus, these conventional braking resistors cannot be switched back into the system bus until they have cooled down sufficiently.
In existing systems, a large resistor is simply switched into conductance with the power system for a predetermined time based upon some discrete event upon the system bus. A single large resistor, however, lacks flexibility and imposes an additional burden on the power system during the time it is coupled therewith if less than the total dissipation is required to respond to the disturbance. Another drawback of using a single resistor is the voltage ripple generated by switching the resistor on or off. Since the resistor should be rated so as to absorb the excessive energy for a worst case scenario, when the resistor is switched on, the system bus voltage will drop significantly, and when it is switched off, the voltage will kick back causing voltage overshoot on the system bus. This voltage fluctuation will impact the power quality and cause EMI (Electro-Magnetic Interference) problems in the system.
Furthermore, a single large resistor has a greater weight than using several dynamic brake resistors in order to dissipate a similar amount of energy from the system bus. This is particular relevant in, for example, an aircraft, where weight reduction is a primary objective. Moreover, a FCAS (Flight Control Actuator System) system must be able to deal with simultaneous regeneration of all FCAS systems or repetitive high voltage transient on aircraft electric power systems.
U.S. Pat. No. 5,396,214 discloses a dynamic braking grid resistor configuration for reducing EMI (Electro Magnetic Impulses) generated by electrical braking of an electric motor. This dynamic braking grid resistor configuration, however, does not provide any means for selectively switching the resistor configuration depending on specific system requirements.